Will the Democrats achieve a 'blue wave' in the midterms?

Nancy Pelosi has predicted a great night for the Democrats, but the jury is very much out.Photograph: Joshua Roberts/Reuters

It’s here: the first general election since Donald Trump won the presidency. For many American progressives, anxiety is rampant that talk of a Democratic victory will collapse into some terrible new reality by night’s end.

Election night could end with the Republicans retaining their majority in the House of Representatives and strengthening their majority in the Senate, despite a historic exodus of Republicans retiring from Congress, widespread disapproval of Trump and the customary strength of the opposition party at this stage in the elections cycle.

Yet some prominent Democrats – starting with the House minority leader, Nancy Pelosi – are predicting a blue “wave” that will deliver a strong Democratic majority to the House. (Predictions of the Senate flipping blue are notably fewer.)

What do we really know about what is going to happen? The short answer is nothing, be patient, it won’t be long now and we’ll know for real.

A slightly longer answer notes that we can point to certain factors likely to be significant on election day that are encouraging for Democrats.

We know that Trump’s approval rating has hovered in the low 40s, a slack tide for hundreds of local Republican candidates. Democrats, meanwhile, have notched a string of unexpected wins in special elections, have attracted an unprecedented slate of first-time candidates, especially women, and have lots of encouraging polling to point to, if they dare.

We know that Democrats need a net gain of 23 seats to gain control of the House, and that they could pull that off by winning as few as about one out of every three “tossup” races – races too close at this point to call. One in three does not seem like such a tall order, especially if one believes polls indicating that a plurality of voters right now prefer Democrats to Republicans.

How many seats would constitute a ‘wave’?

There is no agreed-upon threshold, but 23 seats is not a wave. In 1994, Republicans scored a 54-seat swing; in 2010, Republicans scored a 63-seat swing. Waves that big have been relatively rare; the one before that was in 1948.

If Democrats won a majority of the toss-up races in the House, ended the night with a majority of the 50 governorships and scored a surprise or two in senate races, that could seem wavy, especially if Democrats claimed a major Republican scalp such as Senator Ted Cruz or the Iowa congressman Steve King.

The line between a narrow Democratic victory and a Democratic wave election is thin in the sense that a small difference in the underlying fundamentals of the race – the number of registered Republican voters who happen to vote Democratic that day, for example – could create a big difference in the number of seats Democrats snag.

Elections forecasters see higher probabilities for scenarios in which the Democrats pick up 38 or 39 seats than they do for other scenarios. But because elections are fixed snapshots of a dynamic system, forecasters also countenance scenarios in which Pelosi arrives at the speaker’s rostrum via surfboard, with a majority in the dozens of seats.

The other scenario that forecasters countenance is one in which Democratic voters turn out not to have been as motivated as they seemed, or Republican voters turn out to be more motivated. Maybe voters agree with Trump and credit Republicans for a strong economy, or an unexpected number of voters are motivated by Trump’s racist pre-election anti-immigrant barrage.

In that scenario, a wave never develops, in part because there has been no recent flood of Republicans to wash out: in 2016, nothing much changed around the House, with Democrats picking up six seats.

Republicans maintaining control of the House is well within the margin of error as the CNN forecaster Harry Enten judges it, with a 12.5% chance of happening, as FiveThirtyEight’s Nate Silver sees it. Silver’s final forecast in November 2016 gave Trump a 28.6% chance of winning.

It appears, then, that Republicans have a steeper hill to climb to maintain control of the House in 2018 than Trump did to win the presidency in 2016. If that is any comfort.